The present invention is concerned with spring assemblies utilized to drive the throwing arm of a ball throwing machine that is loaded against a stationary abutment in response to rotation of a central drive shaft.
Various machines have been designed for use in mechanically throwing balls for batting practice and catching in various sports. Such a machine is illustrated and described in our prior application titled "Ball Throwing Machine" filed Mar. 10, 1978, Ser. No. 885,443. Described is a compact lightweight machine for throwing balls along a desired trajectory. It includes a housing with an arcuate track leading inwardly from an opening to an initial ball support station. An abutment is selectively positioned at one of several locations adjacent to the ball support station. A throwing arm is rotatably mounted within the housing and is connected to a spring on the drive shaft. The spring will load in response to rotation of the drive shaft, pressing the throwing arm against the abutment. When the spring is sufficiently loaded, the throwing arm will slip from engagement with the abutment and forcibly move against the ball to eject it from the opening of the housing at relatively high velocity. An energy absorbing provision is made to take up at least some of the momentum of the spring as it moves past the release point between the ball and the throwing arm. The energy absorbing feature is provided in the form of an oppositely wound spring that is positioned on the drive shaft to load against the throwing arm as it moves a normal, unloaded position, thereby preventing stress reversal within the spring that drives the throwing arm.
The spring arrangement for the pitching machine disclosed above is very effective and durable, especially with the added provision of the energy absorbing spring mechanism. However, repeated stresses occur along the length of the throwing arm and with extended use, the throwing arm will fail.
The throwing arm, when under load, takes on the stress characteristics of a cantilevered beam with an anchor point at the abutment and a load produced at the inner end of the arm where it connects to the driving torsion spring. As with all cantilevered beams loaded at one end, strain increases progressively along the length of the arm from a zero point at the load to a maximum load at the anchor point. Therefore, the throwing arm is under twice the strain at the anchor point as it is midway along its length. This concentrated and repeated stress along the length of the throwing arm causes eventual fatigue of the throwing arm spring material, which leads to eventual material failure.
It therefore becomes desirable to increase the effective life of the spring assembly by more evenly distributing loads along the length of the throwing arm.